You can store the resources file on bitrise.io.
In the Workflow Editor select the "Code signing & Files" section,
and upload the archive file in the "Generic File Storage" section.
Bitrise will automatically generate a (time limited, expiring) download URL
for the files you upload here, and will expose the download URL
as an Environment Variable.

If you have your resources on your server in a zip archive, all you have to do
is to add the ZIP resource archive downloader Step to your Workflow,
specify the URL of the ZIP and the destination where the zip's content should be uncompressed.

The source code of your app will be (by default) downloaded into the folder
defined in the $BITRISE_SOURCE_DIR environment variable.

If you want to place the content of your ZIP archive into a folder called myresource
inside your app's source code directory, you can define the extract target folder
(of the ZIP resource archive downloader step) as ${BITRISE_SOURCE_DIR}/myresource,
or ./myresource (as the default working directory is the source code directory).

If you only want to download a single file, you can of course ZIP it up and
use the ZIP resource archive downloader step as described in the previous section,
but there's also a step for single file downloads.

The File Downloader step can be used for this use case. Works very similarly as the
ZIP resource archive downloader step, except it does not require a ZIP file,
it simply downloads the specified file to the location you set.